Ultra-slim nuclear magnetic resonance tool for oil well logging

ABSTRACT

NMR properties of earth formations are determined using a logging device movable in a borehole. The logging device includes a magnet assembly to generate a static magnetic field and an antenna expandable from the surface of the magnet assembly into the borehole toward the borehole wall to increase the magnetic dipole moment of the antenna. The logging device can be lowered or raised through a drill pipe with the magnet assembly being configured to generate no magnetic field while the device is conveyed within the drill pipe. The logging device may also include a side-looking sensor to acquire fast relaxation component of the NMR signals.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates generally to the field of measuring nuclearmagnetic resonance properties of an earth formation traversed by aborehole. More particularly, the invention presents method and apparatusadapted for using in pipe assisted wireline and similar applications,including through the drill bit applications.

Background Art

Various prior approaches have been suggested and implemented formeasuring nuclear magnetic resonance (NMR) properties of earthformations surrounding a borehole to evaluate the earth formations. Mostrecent generation of NMR well logging instruments employs a staticmagnetic field produced by a permanent magnet to align nuclear spinmagnetic moments of protons or other nuclei present in the earthformations. The aligned spin magnetic moment is typically brought intoexcited state by applying an RF magnetic field. RF voltages are inducedin the receive antenna as a result of precessional rotation of nuclearspin axes of hydrogen protons about the static magnetic field withcharacteristic resonance or Larmor frequency corresponding to the staticmagnetic field strength.

Practical wireline NMR downhole tools are represented by U.S. Pat. No.4,717,878 issued to Taicher et al. representing a centralized type tooldesign, U.S. Pat. No. 5,055,787 issued to Kleinberg et al. representinga skid type side looking design with quasi-homogeneous static magneticfield, and U.S. Pat. No. 6,452,388 issued to Reiderman, et al.representing side-looking gradient type design. A permanent magnet usedin all the practical wireline NMR tools generates polarizing magneticfield that aligns nuclear spin magnetic moment. The angle between thenuclear magnetization and the polarizing magnetic field is then changedby applying a pulsed radio-frequency (RF) magnetic field at a frequencycorresponding to the static magnetic field magnitude at a predetermineddistance from the NMR tool. A sequence of RF pulses can be designed tomanipulate the nuclear magnetization in order to acquire NMR relaxationproperty of the earth formation. For the NMR well logging the mostcommon sequence is the CPMG sequence that comprises one excitation pulseand a plurality of refocusing pulses. One of the main challenges of theNMR downhole measurements is to achieve an acceptable signal-to-noiseratio (SNR). Typical SNR for downhole NMR measurements is 3-10 per onemeasurement cycle. For the configurations of the magnets and theantennas of the NMR tools represented in the US patents '878 '787 and'388 the SNR is less for smaller tool diameter.

Pipe assisted wireline method of acquiring borehole data gives uniquecapability of acquiring formation data in difficult well situations, aswell as in high angle or horizontal wells. This method typically requiresmall diameter logging tools capable of being lowered or raised throughthe drill string. Typical tool diameter for this application is about 2inches. There are no NMR logging tools having outer diameter smallenough to be used in the pipe assisted wireline application. A prior artNMR logging tool of this diameter would have an unacceptably small SNR.Therefore there is a need for an ultra slim NMR logging tool withsufficiently high SNR.

Thus known in the art instruments do not give any satisfactory solutionfor an ultra-slim NMR logging tool with sufficiently high SNR. Thereforeit is an objective of the present invention to provide a solution forhigh SNR slim NMR tool suitable for the pipe assisted wirelineapplication.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is an ultra-slim NMR well loggingtool that comprises a magnet to generate static magnetic field, anantenna to generate radio-frequency magnetic field in the volume ofinterest in the earth formations, the magnet and the antenna are adaptedfor lowering or raising through a drill string. The magnet and theantenna have a longitudinal axis of elongation parallel to the axis ofthe drill string. The magnet is magnetized in the directionperpendicular to the magnet axis. The antenna has a closed positionconfiguration while moving the tool within the drill string. The antennahas also an open position configuration when performing measurementsoutside the drill string. The open position configuration of the antennahas a greater magnetic dipole moment. The greater magnetic dipole momentof the antenna enhances the NMR signal and the signal-to-noise ratio.The antenna wires in the open position configuration produceradio-frequency magnetic field which is substantially perpendicular tothe static magnetic field in the volume of interest. Also, in the openposition configuration the antenna wire is located farther from themagnet. This reduces magneto-acoustic ringing, which typicallyaccompanies NMR measurements, especially in case of “inside-out” NMRsensors used in NMR well logging applications. In a preferred embodimentof the invention the magnet has a first configuration realized whilemoving the tool within the drill string and a second configurationrealized when performing measurements outside the drill string. In thefirst configuration the magnet has substantially zero dipole magneticmoment and, consequently, zero magnetic field outside the magnet inorder to prevent the magnet from getting stuck inside the drill pipe dueto the magnetic attraction force between the magnet and the drill pipe.In the second configuration of the magnet it has maximum possible dipolemoment and generate maximum possible magnetic field outside the magnet.In a preferred embodiment of the NMR tool the antenna wires are attachedto a retractable bow spring centralizer which is retracted when the toolis within the drill string and expends into the borehole when the toolis in measurement position.

Another preferred embodiment of the ultra-slim NMR logging tool of thepresent invention is an axially symmetrical tool comprising a first softmagnetic core with a set of coils driven by a DC current to generate aradial static magnetic field in a region of interest. The region ofinterest has a form of elongated cylindrical shell coaxial with the toolaxis. Uniformity of the static magnetic field over the volume ofinvestigation is achieved by adjusting currents in separate coils or acurrent density in one coil. A second magnetic core with at least onecoil is used to generate radiofrequency magnetic field in the region ofinterest and receive NMR signals from the region of interest, theradio-frequency magnetic field direction is substantially perpendicularto the static magnetic field. The second magnetic core is substantiallyelectrically non-conductive.

Another aspect of the present invention is an at least one side-lookingNMR sensor for acquiring fast relaxation component of the NMR signal.The sensor comprises a source of static magnetic field and a source ofradio-frequency magnetic. The static magnetic field and theradio-frequency magnetic field are mutually orthogonal. Both fields areperpendicular to the borehole axis. A magnetic core made of amagnetically permeable material is used as a part of the static magneticfield source and the radio-frequency magnetic field source. Thearrangement used as the radio-frequency magnetic field source can beused also as a receiver of the NMR signal. The sensor is configured as amagnetic head-type device with substantially no parasitic NMR excitationin the borehole. In one embodiment of the side-looking NMR sensor thesource of the static magnetic field is a coil wound around the magneticcore. A plurality of the sensors can be used to enable azimuthalselectivity of measurements.

Yet another aspect of the present invention is a method of measuringproperties of the earth formation at high logging speed or in case ofrelatively short excitation region in axial direction. The methodcomprises employing a first radio-frequency pulse sequence to generatesteady state free precession of nuclear spins and acquire signalproportional to a total amount of hydrogen in porous space of the earthformations and employing a second pulse sequence that uses short trainsof radio-frequency pulses and a short time interval between the trainsto estimate the amount of nuclei having fast NMR relaxation. The fastrelaxation amount is preferably acquired using the side-looking NMRsensor. The method further comprises estimating parameters of the porousspace in the earth formations and characterizing fluids in the porousspace based on the total amount of hydrogen and the amount of nucleihaving fast NMR relaxation.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is best understood with reference to the accompanyingfigures in which like numerals refer to like elements.

FIG. 1 illustrates an exemplary embodiment of a logging instrument usedin pipe assisted wireline application.

FIG. 2 shows a cross sectional view of an exemplary embodiment of aprior art magnet and RF antenna assembly used for NMR well loggingmeasurement.

FIG. 3A, FIG. 3B and FIG. 3C, collectively referred to as FIG. 3, depictrespectively a cross-sectional view of an exemplary embodiment of alogging tool in closed position as conveyed within the drill pipe, across-sectional view of the logging tool in open position as used whenperforming measurement, and a side view of the logging tool in openposition.

FIGS. 4A and 4B illustrate fragments of an antenna assembly.

FIGS. 5A, 5B, 5C collectively referred to as FIG. 5 depict an exemplaryembodiments of the magnet assembly. FIGS. 5A and 5B represent anembodiment with two coaxial magnets in two positions: 1) when the NMRlogging tool is conveyed through the drill pipe (FIG. 5A), and 2) whenperforming measurements (FIG. 5B). FIG. 5C represents an embodiment withcentralizing rollers.

FIG. 6 illustrates an exemplary embodiment of a slim logging tool withlongitudinal RF antenna.

FIG. 7 depicts an exemplary RF pulse sequence to be preferably used withthe embodiment of the slim tool presented in FIG. 6.

FIG. 8A, FIG. 8B and FIG. 8C illustrates an exemplary embodiment of anauxiliary magnet and antenna assemblies (side-looking NMR sensors) forhigh resolution measurement of fast components of NMR relaxation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a borehole 10 drilled into a geological formation 12 to beinvestigated for potential hydrocarbon producing reservoirs. A drillstring 14 extends from a drilling rig (not shown) into the borehole 10whereby the open lower end 15 is disposed in the open lower boreholeportion. An NMR logging tool 16 capable of being lowered or raisedthrough the drill string 14 is suspended in the drill string 14 using awireline cable 18 deployment. The tool is typically lowered down thedrill string and “pumped” out of drill pipe into open hole. The NMRlogging tool in the open hole position is shown at 20. The tool 20 has aretractable bow spring centralizer 22. Instead of the bow spring thetool 20 may have retractable arms or a “car jack” type centralizer. Thetool 20 may be powered by a battery (not shown) and can be provided withan electronic memory (not shown) or other recording medium for storingmeasurement data. The simplified layout of FIG. 1 can represent both aknown in the art logging tool and an embodiment of the presentinvention.

It is to be clearly understood that the embodiment of the logging toolshown in FIG. 1 is only an example. It is within the scope of thisinvention to include different modes of operation: a wireline, includingthrough the bit conveyance (out through the pipe end or through the bit)or memory mode (hanging the tool of the pipe end or off the bit andlogging while tripping pipe). In both cases the tool may be lowered onwireline, by pumping down, or combination of the two.

Turning now to FIG. 2, where a cross-sectional view (the planeperpendicular to the borehole axis) of the NMR logging tool of prior artis presented. The logging tool comprises a magnet 30 and the antenna 32.Magnetization direction 34 of the magnet is perpendicular to the axis ofthe tool, the axis is parallel to the axis of the borehole. Electricalcurrent direction 35 is parallel to the axis of the tool (dots and thecrosses show directions pointing toward and away from the viewerrespectively). A static magnetic field 36 and a radio-frequency magneticfield 38 are mutually orthogonal and have a substantially constantmagnitude at the volume of investigation 40.

It was shown in U.S. Pat. No. 5,712,566 that the signal-to-noise ratiofor the tool configuration shown in FIG. 2 can be presented as follows(the constants are omitted):

$\begin{matrix}{{{SNR} \propto {B_{0}^{1/4} \cdot \left( \frac{B_{1}}{I_{1}} \right)^{3/2}}},} & (1)\end{matrix}$

where B₀ is the static magnetic field in the volume of investigation(NMR excitation volume),

$\left( \frac{B_{1}}{I_{1}} \right)$

is the antenna efficiency defined as a radio-frequency magnetic fieldthat would be generated in the volume of investigation by unit currentin the antenna wire.

Since B₀ is proportional to the cross-sectional area of the magnet and(B₁/I₁) is proportional to the diameter of the antenna assembly(substantially the same as the diameter of the magnet for the toolpresented in FIG. 2) it is clear from the equation (1) that reducingdiameter of the tool presented in FIG. 2 would drastically reduce theSNR, specifically, as the diameter squared. This means that a 2″diameter tool would have approximately 10 time less SNR than a 6″diameter tool making the 2″ diameter tool impractical. Thisconsideration is principally valid for all the practical wireline NMRtools described in the US patents '878 '787 and '388.

FIG. 3A, FIG. 3B and FIG. 3C represent a preferred embodiment of thepresent invention. FIG. 3A shows the NMR tool 20 inside the drill pipe14 while conveying the tool through the drill pipe disposed in theborehole 10. The tool comprises a magnet 41 having magnetizationdirection 44 and an antenna assembly 42A. FIG. 3B and FIG. 3C show thetool when the antenna assembly is expanded radially toward the boreholewall. In order to provide a desired matching between the static magneticfield 46 and the radio-frequency magnetic field 48 the antenna current45 is preferably distributed as shown at 42B. The distribution shouldpreferably approximate a cos(φ) distribution, where φ is the anglebetween the direction of magnetization of the magnet and a directiontoward a particular point on the perimeter of the antenna assembly. In asimplified arrangement of FIG. 3 the antenna current carrying wires areattached to a retractable bow spring 52. A number of bow springs can beused to accommodate antenna wires in order to provide a desireddistribution of the antenna current density. The antenna currents makeclosed loops by running wires around the magnet as shown at 54. Allantenna wires are preferably connected in series. FIG. 3C shows across-sectional view in the plane defined by the direction of themagnetization of the magnet and the direction of the tool axis.

In the open position configuration the antenna wire is located fartherfrom the magnet. This reduces magneto-acoustic ringing, which typicallyaccompanies NMR measurements, especially in case of “inside-out” NMRsensors used in NMR well logging applications.

FIG. 4 shows a fragment of the antenna assembly 42. The fragment shownin two exemplary embodiments FIG. 4A and FIG. 4B. The fragmentillustrates possible layout of the antenna wires attached to one arm ofthe bow spring centralizer 52. In one embodiment a single flat wire 57is attached to the spring 56. FIG. 4B presents another embodiment usinga flexible printed circuit board 58 with multiple wires 59.

FIG. 5 represents possible embodiment of the magnet 41. The embodimentis intended to eliminate or reduce the attraction magnetic force betweenthe magnet and the drill pipe (typically made of a magnetic steel) whilethe tool is conveyed through the interior of the drill string 14 (FIG.1). FIG. 5A and FIG. 5B represent a magnet made of an outer 60A andinner 60B cylinders. While the tool is conveyed within the drill string14 the relative azimuthal orientations of the cylinders is such that thetotal magnetic dipole of the magnet and, correspondingly, the externalmagnetic field is substantially zero. This state of the magnet isrepresented in FIG. 5A. When conducting NMR measurements in the borehole(outside the drill string) the outer and the inner cylinders have thesame direction of magnetization, therefore the magnet 41 generatemaximum possible static magnetic field. FIG. 5C shows an embodiment ofthe magnet where the magnet is centralized in the drill string boreusing rollers 61. The centralized magnet has substantially zeroattraction force to the drill string.

The diameter of the borehole 10 and the position of the tool withrespect to the borehole axis can slightly change during logging thewell. This may cause a measurement error (e.g. in a form of a parasiticrandom modulation of the measured NMR signal) due to change of theeffective area of the antenna. One or more miniature magnetometerattached to each arm of the bow spring centralizer are preferably usedto measure distances from the bow springs to the magnet 41 and makecorrections for the variation of the borehole diameter and the toolposition.

FIG. 6 represents another embodiment of the ultra-slim NMR tool. Thetool has an axial symmetry with the axis 65. The NMR tool has a firstmagnetic core 64 made of a high saturation flux density soft magneticmaterial (e.g. a low carbon steel). Two windings 66A and 66B areenergized by a DC power supply (not shown in FIG. 6) and used togenerate magnetic flux 68A inside the first magnetic core; the flux isdirected toward the center of the first magnetic core as shown in FIG.6. The magnetic flux comes out of the core 64 in its central region asshown at 68B to generate the static magnetic field in the volume ofinvestigation 69. The volume of investigation 69 has a shape of acylindrical shell. The static magnetic field has radial direction in thevolume of investigation. An axial distribution of the current density inthe windings and the axial position of the windings is selected toprovide axial uniformity of the static magnetic field in the volume ofinvestigation 69. A radio-frequency coil 71 and a second magnetic core70 are used to generate a radio-frequency magnetic field in the volumeof investigation. The radio-frequency magnetic field direction issubstantially parallel to the a axis of the tool in the volume ofinvestigation. The second magnetic core 70 is made of a substantiallyelectrically non-conductive soft magnetic material (ferrite or alaminated structure made of thin metal tapes or ribbons that is amacroscopically non-conductive structure). The second magnetic core isused to concentrate the radio-frequency magnetic flux. It also acts as amagnetic shield between the coil 71 and the electrically conductivefirst magnetic core 64. Without the shield the radio-frequency coilquality factor and the radio-frequency dipole moment would besignificantly reduced by the presence of the first magnetic core 64. Itwould be readily understood by those skilled in the art that a reductionof the diameter of the tool and, consequently, the diameter of thesecond magnetic core 70 and the coil 71 will have substantially noeffect on the efficiency of generation of the radio-frequency magneticfield in the volume of interest. This is due to the fact that theradio-frequency magnetic dipole is mainly determined by the magneticflux generated in the magnetic core 70 and also by the fact that thedecrease of the diameter of the core would equally reduce thecross-sectional area of the core and increase its effective magneticpermeability (the latter is primarily determined by the elongation ratioof the core rather than the magnetic permeability of the material usedto make the core).

While the tool is conveyed through the interior of the drill string 14(FIG. 1) the windings 66A and 66B are not energized, therefore there isno attraction magnetic force between the magnet and the drill pipeduring the tool conveyance process.

Reference is now made to FIG. 7 where a method of measuring propertiesof the earth formation at high logging speed or in case of relativelyshort excitation region extent in axial direction. The method comprisesemploying a first radiofrequency pulse sequence to generate steady statefree precession (SSFP) of nuclear spins (described, for example, in P.Mansfield and P. G. Morris. NMR Imaging in Biomedicine) and acquiresignal proportional to a total amount of hydrogen in porous space of theearth formations. An exemplary SSFP pulse sequence comprises phasealternated 90° radio-frequency pulses 72 (shown in the FIG. 7 are theenvelopes of the radio-frequency pulses). The SSFP nuclear magnetizationresponse is shown at 76. The response 76 is the nuclear magnetization(the envelopes of the radio-frequency magnetization patterns) in theplane perpendicular to the static magnetic field direction. The methodalso employs a second pulse sequence that uses short trains of theradio-frequency pulses and a short time interval between the trains toestimate the amount of nuclei having a fast NMR relaxation. The fastrelaxation components of the nuclear magnetization is preferablyacquired using the side-looking NMR sensor described later herein. Themethod further comprises estimating parameters of the porous space inthe earth formations and characterizing fluids in the porous space basedon the total amount of hydrogen nuclei and the amount of nuclei havingfast NMR relaxation.

FIG. 8A, FIG. 8B, and FIG. 8C represent another aspect of the presentinvention: a side-looking NMR sensor for acquiring fast relaxationcomponent of the NMR signal. In one embodiment of the sensor shown inFIG. 8A the sensor comprises a source of local static magnetic fieldrepresented by a magnet 80 and a soft magnetic core 82. The magneticflux of the magnet and the static magnetic flux in the core is shown at84. The magnetic flux and the magnetic field 86 in the volume ofinvestigation 92 in the earth formations is directed perpendicular tothe borehole axis (not shown in FIG. 8A). The borehole axis isperpendicular to the plane of the drawing. A radio-frequency magneticflux in the core is generated by the radiofrequency coil, the two partsof which are shown at 88A and 88B. The radio-frequency magnetic fluxdirection in the core is shown at 90. The radio-frequency magnetic field91 at the volume of interest is perpendicular to the direction of thestatic magnetic field and also perpendicular to the borehole axis (thelatter is perpendicular to the plane of the drawing). The soft magneticcore made of a magnetically permeable material is used as a part of thestatic magnetic field source and as a part of the radio-frequencymagnetic field source. In another embodiment of the side-looking sensorshown in FIG. 8B the source of the static magnetic field is formed by acoil 94 and a magnetic core 96. The static magnetic flux direction inthe magnetic core is shown at 95. The static magnetic field direction inthe volume of interest 104 is shown at 98. A radio-frequency magneticflux in the core is generated by the radiofrequency coil, the two partsof which are shown at 100A and 100B. The radio-frequency magnetic fluxdirection in the core is shown at 101. The radio-frequency magneticfield 102 at the volume of interest is perpendicular to the direction ofthe static magnetic field and also perpendicular to the borehole axis(the latter is perpendicular to the plane of the drawing). In bothembodiment of the side-looking sensor presented in FIG. 8 theradio-frequency coils can be used for generating the radiofrequencymagnetic field in the volume of interest and also to receive nuclearmagnetization signal emanating from the volume of interest. FIG. 8Cshows a side view of the side-looking sensors. Shown at 110 is a part ofthe logging tool in the borehole 112 after passing through the interiorof the drill string. The side-looking sensor 114 is attached to the toolusing a retractable arm 115.

In both embodiment of the side-looking sensor presented in FIG. 8 thesoft magnetic core is made of a magnetically permeable material that ismacroscopically non-conductive (ferrite or stack of thin soft magneticmetal ribbons or tapes separated by insulating layers). The core is usedas part of the static magnetic field generation, the radio-frequencymagnetic field generation and the nuclear magnetization signal receptionsubsystems of the sensor. In both embodiments of the side-looking sensorpresented in FIG. 8 the radio-frequency coils can be used for generatingthe radiofrequency magnetic field in the volume of interest and also toreceive signals produced by X-Y components of the nuclear magnetization

It is important that the sensors presented in FIG. 8 are configured as amagnetic head-type device with substantially no parasitic NMR excitationin the borehole.

A plurality of the sensors can be used to enable measurements indifferent directions (azimuthal selectivity of measurements).

It would be readily understood by those skilled in the art that in theembodiments of the ultra-slim logging tool and the side-looking sensorspresented in FIG. 3, FIG. 6, and FIG. 8 separate coils can be used fortransmission and reception of NMR signals from the volume of interest.

It would be also understood by those skilled in the art that the antennawires (see a preferred embodiment in FIG. 3) placed on different arms ofthe bow spring centralizer could be connected to different receivers inorder to enable directional measurement (an azimuthal selectivity).

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefits of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of invention as disclosed herein. Forexample, to facilitate high signal-to-noise NMR measurements in a rangeof borehole diameters (e.g. from 6 inches to 17 inches range) anembodiment of a conventional wireline conveyance NMR logging tool can bedevised, which uses a slim magnet assembly and a larger diameter antennaassembly as shown in FIG. 3B and FIG. 3C. Bow springs or retractablearms can be used to make the antenna assembly adjustable to expandradially toward the borehole wall and therefore providing a sufficientlylarge magnetic dipole moment of the NMR antenna.

1. An apparatus for measuring nuclear magnetic resonance properties ofearth formations surrounding a borehole, comprising: a logging deviceconveyable through the borehole; a magnet assembly in the logging deviceto generate a static magnetic field; and an antenna assembly in thelogging device for at least one of generating a radio-frequency magneticfield and receiving nuclear magnetic resonance signals; wherein at leastone of the antenna assembly and the magnet assembly has a firstconfiguration and a second configuration, the first configuration beingused while conveying the logging device though the borehole and thesecond configuration being used while conducting nuclear magneticresonance measurements.
 2. The apparatus of claim 1, wherein the secondconfiguration of the antenna assembly has a greater magnetic dipolemoment compared to the dipole moment in the first configuration.
 3. Theapparatus of claim 1, wherein the first configuration of the magnetassembly has a substantially reduced magnetic dipole moment than in thesecond configuration to reduce or eliminate a magnetic attraction forcebetween the magnet assembly and a drill string or surface casing.
 4. Theapparatus of claim 1, wherein the magnet assembly has an axis ofelongation, and the static magnetic field is perpendicular to the axisof elongation.
 5. The apparatus of claim 3, wherein the magnet assemblyhas an axis of elongation, and the static magnetic field is axiallysymmetrical with respect to the axis of elongation; the magnet assemblyfurther includes windings and a current source, the current sourceenergizing the windings when the magnet assembly is in the secondconfiguration and does not energize the windings when the magnet is inthe first configuration.
 6. The apparatus of claim 1, wherein theantenna assembly in the second configuration takes a farthermostposition from the magnet assembly to enable at least one of increase ofthe antenna magnetic dipole moment and reduction of magneto-acousticringing.
 7. The apparatus of claim 1, wherein the antenna assemblyincludes at least one wire attached to a centralizer.
 8. The apparatusof claim 7, wherein the centralizer comprises one of a bow spring and anarm.
 9. The apparatus of claim 1, wherein the antenna assembly comprisesmultiple conductors configured to expand radially toward a borehole wallto produce a desired distribution of the radio-frequency magnetic fieldin the earth formations surrounding the borehole.
 10. The apparatus ofclaim 1, wherein the antenna assembly comprises multiple conductors,each of the multiple conductors being connected to a separate circuitryand a processor is configured to estimate an azimuthal distribution ofthe nuclear magnetic resonance properties of the earth formationssurrounding the borehole.
 11. An apparatus for measuring nuclearmagnetic resonance properties of earth formations surrounding aborehole, comprising: a logging device conveyable through the borehole;a magnet assembly in the logging device to generate a static magneticfield; and an antenna assembly in the logging device for at least one ofgenerating a radio-frequency magnetic field and receiving nuclearmagnetic resonance signals; wherein the antenna assembly is configuredto be adjustable to expand towards the borehole wall to increase theantenna assembly magnetic dipole moment.
 12. The apparatus of claim 11,wherein the magnet assembly comprises a set of rollers to reducefriction between the magnet assembly and the drill string or surfacecasing.
 13. The apparatus of claim 11, wherein the antenna assemblytakes a farthermost position from the magnet assembly to increase theantenna assembly magnetic dipole moment.
 14. A method for measuringnuclear magnetic resonance properties of earth formations surrounding aborehole, comprising: conveying a logging device through the borehole;generating a static magnetic field using a magnet assembly in thelogging device; and generating a radio-frequency magnetic field orreceiving nuclear magnetic resonance signals using an antenna assemblyin the logging device; wherein the antenna assembly is configured to beadjustable to expand towards the borehole wall to increase the antennaassembly magnetic dipole moment.
 15. The method of claim 13, wherein atleast one of the antenna assembly and the magnet assembly has a firstconfiguration and a second configuration, the first configuration beingused while conveying the logging device though the borehole and thesecond configuration being used while conducting nuclear magneticresonance measurements.
 16. The method of claim 14, wherein the step ofusing an antenna assembly further comprises adapting the antennaassembly to have a plurality of current loops to generate, in the secondconfiguration, a substantially axis-symmetrical distribution ofmagnitude of the radio-frequency magnetic field in the earth formationssurrounding the borehole, the radio-frequency magnetic field beingsubstantially perpendicular to the static magnetic field in the earthformations surrounding the borehole to satisfy nuclear magneticresonance excitation conditions.
 17. The method of claim 14, wherein thestep of conveying a logging device further comprises a step of passingthe logging device through one of a drill string and a surface casingfrom a position within the drill string or the surface casing to aposition outside the drill string or the surface casing, the nuclearmagnetic resonance properties being measured when the apparatus is inthe position outside the drill string or the surface casing.
 18. Themethod of claim 15, wherein the step of generating a static magneticfield comprises reconfiguring the magnet assembly from the firstconfiguration with substantially zero magnetic dipole moment when thelogging device is in the position inside the drill string or surfacecasing to the second configuration with maximum dipole moment when thelogging device is in the position outside the drill string or surfacecasing.
 19. The method of claim 14, wherein the step of generating aradio-frequency magnetic field further comprises using a magnetometer tomeasure a distance from a fragment of the antenna assembly to the magnetassembly and make corrections for the variation of the borehole diameterand the logging device position.
 20. The method of claim 14, wherein themagnet assembly comprises a set of rollers to reduce friction betweenthe magnet assembly and the drill string or surface casing.